Simulated moving bed (SMB) chromatography is a continuous chromatographic separation technology. In this technology, many chromatographic columns are connected successively, and the sites for the inlet and the outlet ports are periodically switched along the flow direction of the liquid phase. Therefore, a “simulated” counter-current movement of the solid phase against the liquid phase is achieved, and the mass transfer between the solid phase and the liquid phase is improved, which substantially enhances the chromatographic separation efficiency. Originally, SMB was mainly used in C8 separation in the field of petrochemical industry as well as sugar separation in the field of food industry. SMB is most suitable for the binary separations; thus, it has been successfully introduced into the field of separation of chiral drugs since 1990s and was paid much attention. So far, the principle and the process optimization of the simulated moving bed in binary separations have been studied very deeply and thoroughly.
However, in some cases, there are two or more components other than the target component in the mixture to be treated. According to the elution order on the chromatographic columns used in the SMB process, an impurity eluted before the target component (i.e., an impurity having a retention time shorter than that of the target component) is called a “pre-impurity”, and an impurity eluted after the target component (i.e., an impurity having a retention time longer than that of the target component) is called a “post-impurity”. The “pre-impurity”, the target component, and the “post-impurity” are also called the least, medium and most retained component, respectively, wherein the term “component” indicates a pure substance or a group of pure substances eluted successively. It is particularly common for the separation and purification of natural products, and in such cases, it is difficult to take advantage of a conventional SMB separation method. For example, Professor Bing-chang Lin's research group has encountered this problem during purifying quercetin using SMB, for which they have adopted a process comprising two SMB separation steps: first, the pre-impurities are removed by a first SMB separation, so as to obtain a mixture comprising quercetin and the post-impurities; then, quercetin, as the medium retained target component, is separated from the mixture of quercetin and the post-impurities by a second SMB separation (Li-hua Zhang, Li Juan Gao, Bing-chang Lin. Purification of Quercetin with a Simulated Moving Bed Chromatography, Journal of Anshan Institute of Iron and Steel Technology, 2002,25 (2) : 108-111). However, such a to process is much complicated in application in that the two SMB separation steps cannot be simultaneously carried out on a single device, and the second SMB separation must be carried out after the first SMB separation is completed, which not only increases the consumption of solvents and the wear of the devices, but also involves cumbersome procedures and increases production costs. Therefore, more reasonable solutions are expected to address this issue.